(1) Field of the Invention
This invention relates to a polyurethane emulsion, a sheet-like porous material and a production process of the porous material. It is a primary object of this invention to provide a sheet-like porous material which is excellent in various properties such as surface smoothness, water repellancy (withstandable water pressure), stain resistance, washability, mechanical properties and vapor permeability and is also good in hand and feel.
(2) Description of the Related Art
Numerous sheet-like porous materials composed of a polyurethane resin and suited as a natural leather substitute have heretofore been known. A number of processes has also been known as their production processes. These processes may be divided roughly into wet processes and dry processes.
These processes are each accompanied by both merits and demerits and dry processes are superior from the standpoint of productivity. As such dry processes, there are known those described in Japanese Patent Publication Nos. 4380/1973 and 8742/1973, Japanese Patent Laid-Open Nos. 41063/1976, 66961/1979 and 68498/1979, etc.
Although sheet-like porous materials having excellent vapor permeability can be provided by these known processes, these sheet-like porous materials has a porous structure and corollary to this, is accompanied by drawbacks that they are inferior in surface smoothness, hand and feeling and are prone to staining.
These sheet-like porous materials contain through pores in order to have moisture permeability. As a result, they are accompanied by a further drawback that penetration of external water is easy and when they are used on a rainy day or the like, water penetrates to the inside and the inside is thus wet.
As a method for solving such drawbacks, it is widely practised to incorporate so-called flexibilizer and water repellant, such as fluorocarbon compound and fluorine compound, in the porous layer. Such flexibilizer and water repellant are compounds having a relatively low molecular weight and have poor compatibility with polyurethane resins. They hence tend to bleed out to the surface of the porous layer, thereby causing a staining problem that the surface becomes sticky and more susceptible to dust deposition.
Furthermore, the flexibilizer and water repellant are washed away when the sheet-like porous material is washed repeatedly, leading to a problem in washability that the properties imparted by the flexibilizer and water repellant, such as surface smoothness, hand and feeling, water repellancy (withstandable water pressure) and stain resistance, would be lost.
There is hence an outstanding demand for the development of a sheet-like porous material which has good surface smoothness, hand and feeling and excellent withstandable water pressure, stain resistance and washability in spite of its porous structure.